Weathering, Erosion, & Deposition The surface features of the Earth change as a result of observable physical and chemical processes
Weathering and Erosion This is a monument called Cleopatra’s Needle. It was carved in Egypt around 1450B.C. The sides are carved with hieroglyphs, the writing of ancient Egypt. It stood in the dry, hot Egyptian desert for over 3000 years. During that time, the hieroglyphs remained distinct.
Weathering In 1800, the monument was moved to New York City. Almost immediately, the hieroglyphs began to fade. In only a few years in the wet and variable climate of New York, the Egyptian writing became indistinct!
Erosion Cleopatra’s Needle was carved from granite, a hard tough, crystalline rock. Although it is tough, granite is changed by the atmosphere. Some of the minerals that make up granite change to clay. Chips and flakes of minerals break away from the granite surface.
The Cycle of Changing Earth’s Surface Weathering, erosion, and deposition act together in a cycle that wears down and builds up Earth’s surface Weathering-wears down the Earth’s surface Erosion-moves the Earth’s surface (acts as a bridge between the weathering and deposition) Deposition-builds up the earth’s surface
Weathering The breaking down and/or wearing away of earth materials (rocks) into smaller pieces Must occur before erosion can happen No movement is involved
Physical aka Mechanical Weathering Breaks rocks down into smaller pieces (nothing changes but the size) Sediments: naturally-occurring material that is broken down by processes of weathering; small/tiny pieces of rock Sediments vary in sizes and are named depending on their size
Weathering Physical Common weathering processes: Frost action Thermal Expansion and Contraction (wetting and drying) Biological Activity Exfoliation Abrasion
Types of Mechanical Weathering Thermal Expansion & Contraction (Wetting & Drying) Especially effective at breaking up rocks that contain clay. Clays swell up when wet and shrink when dry. Causes rocks that contain clay, such as shale, to fall apart.
Types of Mechanical Weathering Frost action or Ice Wedging: Water takes up about 10% more space when it freezes. Expansion puts great pressure on the walls of a container. Water in the cracks of rocks wedges the rock apart when it freezes. Often occurs in places where temperatures vary from below and above the freezing point of water.
Abrasion These mountains in Salt Lake City were weathered by abrasion. Abrasion is the grinding action of other rock particles due to gravity or the motion of water, ice or air. Most often seen in running water, the beach, or the desert
Types of Mechanical Weathering Action of plants and animals (biological activity) Larger trees and shrubs may grow in the cracks of boulders. Ants, earthworms, rabbits, woodchucks, and other animals dig holes in the soil. These holes allow air and water to reach the bedrock and weather it.
Types of Mechanical Weathering Granite exfoliation
Exfoliation Occurs when thin layers of rocks break off in sheets
Chemical Weathering This cave was formed by slightly acidic water dissolving the limestone Occurs when chemical reactions dissolve or change the minerals in rocks (rusting, acid breakdown, etc) Occurs most often in warm and humid climates
Chemical Weathering Breaks down rock through chemical changes. The agents of chemical weathering Water: dissolves rock over time Oxygen: combines with iron to form rust (oxidation) Carbon Dioxide: dissolves in water to form carbonic acid Living Organisms: plant roots secrete acids Acid Rain: from the burning of fossil fuels
Living Organisms Lichens that grow on rocks produce weak acids that chemically weather rock.
Oxygen Iron combines with oxygen in the presence of water in a processes called oxidation. The product of oxidation is rust.
Carbon Dioxide Dissolves easily in water. Forms a weak acid called carbonic acid The same compound that is in carbonated drinks. Carbonic acid easily weathers limestone and marble.
Acid Rain Compounds from burning coal, oil and gas react chemically with water, forming acids. Carbon dioxide and sulfur compounds released by industries unite with water in the atmosphere to form acid rain. Increasing amounts of acid rain in the environment increase the rate of chemical weathering. Acid rain causes very rapid chemical weathering.
Physical and Chemical Weathering Rate of weathering depends on: Climate—warm and humid is faster Size of the rock particle Mineral composition—determines how fast a rock weathers Time—most important factor Given enough time all rocks with weather Rock Type—rock that is permeable (has holes that let water in), weathers faster than rock that is impermeable. Weathering creates either sediment or soil Classified by size—sand, silt, or clay
Check for Knowledge How does the size of the rock particle affect weathering? How does the mineral composition of the rock particle affect weathering? Why is time the most important factor?
Lets compare Please draw a Venn diagram and compare chemical and physical weathering. How are they similar/different?
Erosion The movement of weathered earth materials by moving water, wind, gravity or ice Ex: waves causing beach erosion (shoreline of Texas is disappearing) running water Ex: sand storm wind Ex: rain causing a mudslide running water and gravity Ex: landslide gravity Ex: rain carrying soil and gravel down a driveway run ning water and gravity
What are the 4 main ways water erodes? Runoff Stream Erosion Wave Action Glaciers
Erosion The Bernard Glacier in Alaska's Saint Elias Mountains looks like a huge alpine highway. Glaciers are slow but highly effective shapers of the land, essentially carrying away anything in their path—from soil and rocks to hills and even the sides of mountains
Why are glaciers called dirty snowballs? What is glacial till? Glaciers are called dirty snowballs because as they move across the land they carve up the earth’s surface collecting rocks and sediments with the snow. Glacial till is the jagged uneven particles the glacier deposits as it retreats (melts)
Mass Wasting and Creep Mass wasting—downhill movement by gravity Potential increases with slope Causes rock slides, slump, mud slides, or avalanches
Landslides Landslides- AA geological phenomenongeological phenomenon Includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments.
Slump A steep hillside shows a spoon-shaped depression in which material has begun to slide downhill. Occurs along a distinct fracture zone, often within materials like clay that may move rapidly downhill. In some cases the slump is caused by water beneath the slope weakening it.
Soil Creep Can be almost undetectable. Normally found on weathered mountain slopes. Gravity forces the material to slowly slide downhill, and eventually the signs of creep become evident. Tree trunks curve upward as creep caused them to tilt downhill Fence posts and hydro poles may also begin to lean downslope
Why care about erosion? Human activities such as construction increase the rate of erosion 200, even 2,000 times that amount. When we remove vegetation or other objects that hold soil in place, we expose it to the action of wind/water and increase its chances of eroding The loss of soil from a construction site results in loss of topsoil, minerals and nutrients, and it causes ugly cuts and gullies in the landscape. Surface runoff and the materials it carries with it clog our culverts, flood channels and streams. Sometimes it destroys wildlife and damages recreational areas such as lakes and reservoirs.
Bare Slopes vs. Vegetation Stabilized Slopes Mudslide danger Loss of topsoil Clogged storm drains Flooding problems Expensive clean up Eroded or buried house foundations Soil in place Less clean up Minimum erosion Protection for house foundations
Deposition The process of eroded earth materials (sediments) being deposited in a new location Ex: delta-where sediment is deposited where a river flows into an ocean or lake builds up a landform Ex: sand dune-a deposit of wind blown sand
Process of Deposition When sediments are released after being transported Usually when velocity decreases Factors affecting deposition: Particle size—smaller, the slower the rate Shape—more spherical, the faster the rate Density—more dense, the faster the rate
Process of Deposition Grade bedding aka Vertical Sorting Velocity of transporting agent reduced quickly Sediments of mixed sizes Size of sediment increases with depth Horizontal sorting Velocity decreases slow rate over long distance Larger first, smaller last—mouth of river
Erosion and Deposition Meanders are S shaped curves formed by weathering of one bank of a river and deposition of the weathered sediment on the opposite bank of the river. This creates a loop-like bend in a river.
Notes quiz 1) Deposition is a) the movement of sediment or rock b) the settling out of sediment or rock c) the breaking down of sediment or rock 2) A meander is a) a small river that runs into a larger river b) a loop-like bend in a river c) a lake that has been cut off from a river
3) Name 3 methods of erosion control a) b) c) 4) What are riparian buffer zones?